Methods are provided that allow high-level protocol drivers to cancel transmission requests that have been sent to low-level protocol drivers. Transmission requests are assigned cancel identifiers. Because one original request may be divided into several packets for transmission, the same cancel identifier is given to every packet that derives from the original request. High-level protocol drivers can request cancellation of all pending requests whose cancel identifiers match the one indicated. Cancel identifiers generated by different high-level protocol drivers are mutually distinct so one protocol driver cannot inadvertently cancel requests made by another. The protocol driver may divide its requests into logical flows and assign the same cancel identifier to all requests within one flow. Then, the protocol driver may cancel all pending requests within one flow while allowing requests in other flows to proceed to transmission. An application can use these methods to cancel requests sent to high-level protocol drivers.